The present invention relates to a method for selective network monitoring for switchgear. In addition, the present invention also relates to the associated arrangement for implementing the method.
In power distribution installations, one of the most important disruptions occurring in practice is a short circuit in subdistribution networks which leads to short-circuit currents which are generally more than one order of magnitude above the rated currents in the network. Not only can such short circuits lead to local damage due to the arc that is generally produced in the event of a short circuit, but also they impair the security and functioning of the superordinate network. For this reason, power circuit-breakers are used in subdistribution networks, in order, in the event of disruptions in the network, to isolate the affected part of the network, i.e. the subdistribution board with loads in this respect, from the remaining region and, in this way, ensure unimpaired functioning of the overall system. These power circuit-breakers have the task of interrupting the short-circuit current after only a few current half-cycles, in order to minimize any damage and impairment of the network that occur. The power circuit-breakers generally comprise a stationary contact and a contact that can be moved relative thereto, which open a previously closed electric circuit by mechanically separating the contact elements.
In exceptional cases, failure of the power circuit-breaker may occur when, by way of example, the switching contacts have reached the end of their service life. In this case, a superordinate power circuit-breaker must isolate a correspondingly larger part of the subordinate network from the rest of the network. Problems that are posed here involve identifying the failure of the subordinate power circuit-breaker with a high degree of reliability and very early on, in order that the superordinate power circuit-breaker is triggered with a sufficiently short time delay. Especially in the case of vacuum circuit-breakers, when so-called late failures occur, i.e., events in which the power circuit-breaker initially separates successfully but then fails due to an arc restriking, diagnosis of the switching behavior is very important in order to lead with certainty to the triggering of the superordinate power circuit-breaker.
In practice, conventional methods which identify a short circuit on account of the current amplitude in the network and, if appropriate, generate a corresponding signal for triggering the superordinate power circuit-breaker. According to this method, it is generally necessary to effect measurement over a number of current half-cycle durations in order to obtain a sufficiently high signal-to-noise ratio in particular with respect to false triggering. This method has the disadvantage thatxe2x80x94 in particular in the event of failure of the subordinate circuit-breakersxe2x80x94 the short-circuit current has to flow for a number of half-cycles in order to be able to be identified with certainty. This measure is necessary in order to reliably distinguish operational overcurrents from short circuits. This means, however, that damage may already be produced at the location of the short circuit and the disturbance may propagate widely through the power distribution network.
Furthermore, early identification of a short circuit by simultaneous analysis of the rate of current rise in addition to the current amplitude is described in U.S. Pat. No. 4,811,154. A method for early identification of a short circuit by means of digital algorithms is also described in etz, Vol. 112 (1991), pp. 718-722. Early identification of a short circuit is thus possible as early as in the rising part of a half-cycle with the instantaneous current still being comparatively small. However, in this case, too, the failure of a power circuit-breaker can be detected only in combination with the detection of an inadequately long arc duration in the relevant circuit-breaker. Especially in the case of multipole switching devices, it is therefore necessary to concomitantly measure the arc duration in all poles. Corresponding diagnosis as well as the early identification of a short circuit itself are therefore very complicated and cost-intensive. In particular, late failures cannot be identified until they occur and, for this reason, at the very least cannot be predicted.
An object of the present invention, therefore, is to provide a method and an associated arrangement which enables improved selectivity in the network monitoring for switchgear.
This object is achieved by providing the following features:
electrical post-arc currents are detected after current zero,
the electrical post-arc currents are compared with a predetermined limit value, and
if the limit value is exceeded, a signal is generated for triggering a superordinate power circuit-breaker in the switchgear.
In the associated arrangement in which the network of a switchgear has at least one superordinate power circuit-breaker and a plurality of subordinate power circuit-breakers, a device is present which monitors the subordinate power circuit-breakers and, if appropriate, triggers the superordinate power circuit-breaker. For this purpose, the device comprises units for state identification and current measurement including the detection of the post-arc current of the power circuit-breakers, on the one hand, and a release for the superordinate power circuit-breaker.
The solution according to the present invention is based, therefore, on the measurement of the post-arc currents using structural elements of the power circuit-breakers. This is possible, in particularly, using the vapor shields in vacuum circuit-breakers. The level of the post-arc currents is a measure of the dielectric recovery of an interrupter following successful disconnection of a short-circuit current at the end of a current half-cycle. Using an extensive series of experiments, it has been possible to demonstrate that, above an experimentally confirmed, sharply defined limit of the post-arc current after the current zero crossing, even if disconnection is initially successful, failure of the relevant interrupter must be expected with a high probability. On the other hand, a reliable isolating behavior is ensured below this limit. The absolute position of this limit is dependent on the design but, specifically in the case of vacuum interrupters, lies in a range of from about 5 to 15 A which is readily accessible to measurements.
If the present invention is applied to vacuum interrupters, in particular, a prediction regarding the time response in the case of interrupters with a shield that is linked on one side, i.e., with an electrical connection between the vapor shield and one of the two electrodes, is also made possible, even if the interrupter does not fail until after a few 100 ms due to a late failure.